One of relatively few reactions that takes place directly between two solids at roo temperature is

A reaction between two pure solids is very rare. For a reaction to occur, the molecules must interact with each other. Since molecules of solids are very compact, they don't have the liberty to collide with one another. However, this is made possible by hydrates of solids. An examples would be CuSO₄·5H₂O. In this case, they have a an intermediate liquid to react them together.

Alinear relationship would look like a straight line on the graph. it could have a positive negative or 0 slope. so it could be a line potentially going in any direction but it must be perfectly straight.